The present invention relates to a television receiver, and more particularly to an apparatus which identifies the line number of a television signal.
In order to utilize the Vertical Interval Test Signal (VITS) or the Vertical Interval Reference Signal (VIRS), a line recognizer is necessary which identifies the line number on which specific signals are placed. As shown in FIG. 1, a well known circuitry for above use consists of a integrator (1), a threshold circuit (2), a one-shot multivibrator (4) and a counter (5). A sync pulse, which is separated from a television signal by a sync separator (not shown in FIG. 1) is integrated by said integrator (1). The output signal of said integrator (1) is composed of the vertical sync pulse whose waveform is illustrated in FIG. 1, and fed to said threshold circuit (2). Said threshold circuit (2) detects a specific portion of the integrated sync signal which has a larger amplitude than a predetermined threshold level which is determined by the potentiometer (3). The output signal of said threshold circuit (2) triggers said one-shot multivibrator (4) which produces so-called "Counter Enabling Pulse" on its output terminal. The enabling pulse controls operation of said counter (5).
HIGH state of said enabling pulse, for instance, inhibits action of said counter (5) and LOW state allows said counter (5) to count the number of horizontal retrace pulses applied to another input terminal of said counter (5). As said one-shot multivibrator (4) produces a negative-going pulse which has a specific time duration after being triggered by a leading edge of the output pulse of said threshold circuit (2), said counter (5) counts the number of said horizontal retrace pulses behind said leading edge of the output pulse of said one-shot multivibrator (4). Said counter (5) produces the line identification pulse which corresponds to the desired line interval on which VITS or VIRS are placed. After specific time duration sufficient to line identification of said counter (5), said one-shot multivibrator (4) turns said enabling pulse HIGH which inhibits operation of said counter (5). Said counter (5), therefore, continues to be quiescent until next arrival of the vertical sync pulse of the next field.
This prior art requires adjustment of said predetermined threshold level in assembly lines, because said integrated vertical sync pulse has variations in amplitude or waveform due to variations of electrical components or source voltages. In addition, this circuit configuration is not suitable for integrated circuit design because said integrator and said one-shot multivibrator requires many capacitors which cannot be integrated in a chip of integrated circuits. These capacitors increase pins around the integrated circuit and therefore raises IC's price.